How to think about autism risks

March 29th, 2014, 8:57pm by Sam Wang

For those of you coming here from the NYT, welcome! If you’re interested in drilling into the scientific literature, I’ve archived some articles here. Note: if you’re having trouble getting the links from the NYT piece to work, the link above has all the correct files. Basically the NYT site messed up the “~” character in the URL. Soon I’ll document the calculations behind the risk ratios.

In my article in the Sunday New York Times on how to think about autism risks, I apply meta-analytical techniques to autism research literature. It’s nearly impossible to get a good overall perspective from news reports. However, I provide a way to look at it all at once. My secret decoder ring takes the form of risk ratios. Check it out.

As stated in the article, by far the largest risk is genetic. In comparison, the measured impact of environmental risks ranges from nonexistent to small, unless you work directly with chemicals in a factory. The small risks might actually be due to stress. For more information, check back later – or see PubMed, which allows you to search the vast research literature on autism.

11 Comments so far ↓

Dr. Wang, can you honestly tell me that injecting my newborn son in 1999 with 12.5 mcg of mercury on the day of birth and then at 2, 4 and 6 months vaccinating him with the recommended schedule that exposed him with a dose of mercury that was more than the RDA for a 225# man, had nothing to do with his developmental delays (AKA autism)? Breathing mercury can cause autism but injecting it is no problem? As an advanced practice nurse, I don’t buy it. Exposing a developing brain to chemicals and toxins causes developmental delays. Common sense should tell you that and there are studies that prove it.

Thanks for getting this into the NY Times! I find it amazing that there are so many people out there who do not understand the difference between “someone wrote something on the internet” and “paper published in a well-known, anonymous-peer-reviewed journal”.

(Although, to be fair, the vaccination stuff was published, but then retracted. And the author of the fraud was punished.)

I remember we had discussed uncertainties on these numbers. It might be useful to state some of these “no statistically significant risk” as “consistent with zero to extremely high precision (XX sigma)”. Especially the vaccination one.

Also, we had a discussion of correlations. Parent older than XX could well correlate with parent waited until they finished advanced (STEM) degree, which your own study on Princeton undergrads showed correlated with relative who has autism spectrum disorder.

I agree, there is a long story to be told behind each risk ratio, including confidence intervals. I will certainly work on that.

Regarding MMR, the median risk ratio from the Cochrane Report is 0.92. If I recall, 8 out of 10 studies had risk ratios listed. The 2-sigma confidence interval is probably around 0.1. I’ll be sure to calculate that.

There also is a difference between mercury in vaccines (thimerosal, which has been phased out in most vaccines and decomposes to ethylmercury) and mercury in the environment (methylmercury; extremely toxic http://www.usgs.gov/themes/factsheet/146-00/). Ethylmercury also is much less toxic than methylmercury and does not bioaccumulate.

A reader has raised a concern about SSRIs, which are shown on the chart as being not statistically significant. I’ll make a note here, to be expanded later into a full post.

There are four studies on the subject of SSRIs and autism, the most recent ones being largest and analyzing 4,000-5,000 cases. Statistically, we should give the most credence to these large studies. They show no statistically significant association.

One, by Hviid et al., gives a “rate ratio” of 1.2, not significant and furthermore similar to the rate among those who used before pregnancy.

The other, by Sorensen et al., has a sibling control that cleverly rules out errors in diagnosing maternal mental illness. It gives a hazard ratio of 1.1, not significant.

Finally, I note that the largest risk ratio, by Croen et al., had only a few hundred cases. Even there the rate was the same between mothers who used SSRIs during pregnancy, and those who used SSRIs before pregnancy.

Re studying these risk factors in animals: that’s the general game plan among animal researchers, for any disorder. One first step might be to sort out which factors are prime suspects in humans. I would personally put stress signaling pathways higher on the priority list than they are now!

The less than 1.0 risk was, as I recall, true in several studies and in fact may be true. What it may illustrate is not that vaccines are protective but the result of these not being prospective randomized studies. Some parents who are MMR vaccine refusers may be more anxious (and thus refusing) because they are keying in on something being off about their child that they either have not or cannot articulate but that precedes an autism diagnosis. Or the genes that predispose to autism may, in lesser gene loads present in parents, predispose to a personality type which lends itself to becoming a refuser.

Also, true in “some” is far from “a signature.” Autism may be a common phenotype of multiple etiologies. To those who know of chaos theory one way to look at it is as one attractor basin in the massively nonlinear complex system that is the human brain.

Also, there have been about a dozen studies with 100-fold more kids that show MMR has no effect on the risk of autism. In science, you’re not allowed to ignore such a vast body of evidence and make up your own story. You should be ashamed.